1. Field of the Invention
The present invention relates to an electrostatic recording head used in an electrostatic recording apparatus, and to a method of manufacturing such electrostatic recording head.
2. Description of the Related Art
An electrostatic recording head of the multiple electrode type is used in an electrostatic recording apparatus in which an electrostatic latent image is formed at a high speed on a recording medium having a charge holding surface and a conductive layer, by applying image signals from a computer, electrographic or the like, through the electrostatic recording head to the recording medium, and a toner is used to develop the electrostatic latent image into a visible image. Such an electrostatic recording head is known from, for example, JP-A-53-20929, JP-A-56-110959, JP-A-56-122056, and co-pending U.S. application Ser. No. 556,728 filed on July 25, 1990 and entitled "Electrostatic Recording Head and Method of Making the Same".
FIG. 13 is a perspective view showing one of conventional electrostatic recording heads, FIG. 14 is an enlarged view showing a tip portion of the recording head of FIG. 13, and FIG. 15 is a diagram showing a part of the fabrication process of the recording head. The electrostatic recording head is provided with a substrate 100 and a plurality of recording electrodes 101 formed on the substrate 100. The recording electrodes 101 are formed by etching a thin film on the substrate in accordance with a predetermined pattern, and the thin film is deposited on the surface of the substrate 100, for example, by electroless plating. In this case, in order to improve the resolution of a recorded image, the recording electrodes 101 are formed so that the gap between adjacent electrodes on one surface of the substrate and an electrode on the other surface of the substrate face each other.
As shown in FIG. 15, the substrate 100 provided with the recording electrodes 101 is inserted into a mold 105, and thermosetting epoxy resin 102 is introduced into the mold 105 so that the substrate 100 is surrounded with the epoxy resin. After having been hardened, the epoxy resin 102 is taken out of the mold 105, and the tip of the epoxy resin is polished together with the substrate buried therein to obtain the electrostatic recording head shown in FIG. 13. As mentioned above, the substrate 100 is covered with the epoxy resin 102. Thus, the substrate 100, specifically, the tip portion of the substrate 100 which is put in slidable contact with recording paper, is protected by the epoxy resin 102.
In the conventional electrostatic recording head, however, the epoxy resin is put in direct contact with the substrate, and thus there arises a problem that the electric corrosion resistance is low.
In particular, the epoxy resin 102 shanks through hardening processing. Accordingly, the epoxy resin 102 which has been hardened, may separate from the substrate 100, and a gap may be produced between the epoxy resin 102 and the substrate 100. Further, when a high voltage is applied to the recording electrodes 101 for a long time, the epoxy resin existing between adjacent recording electrodes is degraded, so that an offset may occur between the surface of the epoxy resin 102 and the top edge of the substrate 100. When such an offset is generated, the recording electrodes may be deformed due to mechanical slippage between the recording head and the recording sheet during recording operation, resulting in short circuit between adjacent recording electrodes 101. Further, the air moisture may permeate into the head through the offset. When a high voltage is applied to the recording electrodes 101 for a long time in such a state, a phenomenon called "electrochemical migration" occurs. That is, copper ions from the recording electrodes 101 made of copper permeate into the epoxy resin existing between adjacent recording electrodes, which may result in a short circuit between adjacent recording electrodes. Further, since the tips of the electrodes are no more protected by the epoxy resin, the degradation of the electrodes is accelerated so that it may be difficult to ensure excellent quality of recorded image.
The mold resin 102 shrinks and deforms from the semi-fluid state when the resin 102 is introduced into the mold 105 to a solid state after hardening processing. When such shrinkage is generated, a large stress is applied to the substrate having the recording electrodes, and the substrate may be bent. Further, even if the straightness of the substrate 100 is only slightly deteriorated by the above bend, the picture quality of a recorded image will be degraded. Specifically, in a case where the width H of the electrostatic recording head is as large as 90 cm, the substrate is readily bent by the shrinkage of the mold resin 102.
Further, such shrinkage of the mold resin 102 damages the recording electrodes 101, and deviates the distance between adjacent recording electrodes from a predetermined value. Such defects are found after the mold resin 102 has been filled to cover the substrate 100 and hardened. Accordingly, it is required to discard the whole of the electrostatic recording head as a defective. Thus, the manufacturing yield is low, and the manufacturing cost is increased.
Further, another conventional electrostatic recording head is known in which, as shown in FIG. 16, wire electrodes 103 having a predetermined positional relation are used in place of the thin-film electrodes. In a case where this recording head is fabricated, also, the above-mentioned problems will arise.